Glaucoma is one of the leading causes of blindness, resulting from the loss of a particular type of retinal cells, more specifically, retinal ganglion cells (RGC). Axons of the ganglion cells project out of the eye to form the optic nerve. With the loss of ganglion cells, the optic nerve which connects the retina to the brain is gradually destroyed, and results in blindness if the disease is not treated early enough.
Although typical signs of glaucoma include a scotoma as well as a "cupping" of the optical disc, by the time such signs are detected, treatment is unlikely to be successful. While glaucoma is typically associated with an elevation in a patient's intraocular pressure, testing for such an elevation is generally unreliable. This is so, since an elevated intraocular pressure typically occurs transiently in the morning and evening, or may not even be exhibited by some patients.
As such, an evaluation of the patient's visual field has long been the method used for the clinical diagnosis of glaucoma, as well as other pathologies. For example, in so-called "white-on-white" perimetry, a white test object of varying contrast is displayed against a white background at different points in the patient's visual field. The characteristic locations where the test object is undetected for a particular contrast allow clinicians not only to diagnose, but also to determine the severity of the glaucoma. A drawback, however, to this approach is its lack of sensitivity, typically detecting glaucoma only after 30-50% of retinal ganglion cells have been lost or destroyed.
Efforts to improve the sensitivity of the above latter approach have focused on the physiology of retinal ganglion cells. For example, in "short-wave" automated perimetry (SWAP), a blue rather than a white test object is displayed within the patient's visual field, and against a yellow background. The characteristic locations where the test object is undetected for a particular contrast are used to effectively screen patients for glaucoma damage.
Recently, another approach using the phenomenon of so-called "frequency doubling" has also been used for the early detection of glaucoma. See, for example, U.S. Pat. No. 5,065,767, which is incorporated herein by reference. As shown in FIG. 1, in this latter approach, a sinusoidal grating pattern 10 consisting of light and dark bars or striations 20, 30, respectively, is modulated at a temporal frequency between 10 and 50 Hz. That is, the bars are contrast modulated in a sinusoidal fashion from white through gray to black at about 10 to 50 times a sec. At such frequencies, typically about 40 Hz, the grating pattern is perceived by patients to have double the spatial frequency. For a discussion on this phenomenon, see, for example, D. H. Kelly, "Frequency Doubling In Visual Response," J. Opt. Soc. Am., 56:1628-33 (1966); and D. H. Kelly, "Nonlinear Visual Responses To Flickering Sinusoidal Gratings," J. Opt. Soc. Am. 1051-55 (1981).
Patients suffering from glaucoma typically require twice the contrast level between the white and black bars before observing the above frequency doubling phenomenon. This phenomenon is now understood to be a non-linear visual response of the eye. Importantly, this difference in visual response between patients with normal vision and those suffering from glaucoma is used to detect the disease at an earlier stage.
In the above referenced related issued patent, still yet another psychophysical visual test was proposed for testing a person for glaucoma and other diseases which appears to have greater sensitivity. This visual test was based on the discovery that the frequency doubling phenomenon noted herein above was also discovered to be produced by color visual stimuli. More specifically, it was discovered that a visual stimulus consisting of a grating pattern with alternating colors also produced the frequency doubling phenomenon. That is, the colors were preferably of the same luminance or intensity level, i.e., isoluminent, but each grating alternates from one color to another, such as from blue to yellow, and vice a versa.
More specifically, there is shown in FIG. 2 a color visual stimulus 40 consisting of two circular gratings 50,60, here having two spatial cycles. Although the luminance level remains constant, the color of each grating alternates preferably between two colors at a frequency f.sub.s of about 10-50 times a sec. That is, each grating switches back and forth between the two colors at a desired frequency, here the complementary color pair of blue and yellow. At any instance in time, however, the "saturation" of each color varies sinusoidally radially inward, ranging from a maximum to a minimum.
When the colors in visual stimulus 40 are alternated at a frequency between 10-50 Hz, the frequency doubling phenomenon causes four cycles to be perceived, instead of two, if the visual stimulus is observed. Persons suffering from eye disorders, such as glaucoma, however, find it more difficult than normal people to observe this frequency-doubled stimulus. Observation thresholds for this stimulus can be measured by reducing the "saturation" levels of the colors until the visual stimulus disappears. Persons suffering from glaucoma and other diseases will be unable to detect the visual stimulus as the saturation levels are reduced.
Although the above latter color frequency doubling visual test performs well, it would still be desirable to have an alternative visual test which may have a greater sensitivity, so as to detect eye diseases, particularly glaucoma, at its earliest possible stage.